Three-dimensional memory devices and fabrication methods thereof

ABSTRACT

Embodiments of a method for forming three-dimensional (3D) memory devices include the following operations. First, an initial channel hole is formed in a stack structure of a plurality first layers and a plurality of second layers alternatingly arranged over a substrate. An offset is formed between a side surface of each one of the plurality of first layers and a side surface of each one of the plurality of second layers on a sidewall of the initial channel hole to form a channel hole. A semiconductor channel is formed by filling the channel hole with a channel-forming structure. The semiconductor channel may have a memory layer including a plurality of first memory portions each surrounding a bottom of a respective second layer and a plurality of second memory portions each connecting adjacent first memory portions. Further, the plurality of second memory portions is removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is continuation of International Application No.PCT/CN2019/093455, filed on Jun. 28, 2019, entitled “THREE-DIMENSIONALMEMORY DEVICES AND FABRICATION METHODS THEREOF,” which is herebyincorporated by reference in its entirety. This application also claimspriorities to Chinese Patent Applications Nos. 201910248967.4,201910248617.8, 201910248601.7, 201910248966.X, and 201910248585.1, eachfiled on Mar. 29, 2019, all of which are incorporated herein byreference in their entireties. This application is also related to U.S.application Ser. No. 16/541,137, filed on even date, entitled“THREE-DIMENSIONAL MEMORY DEVICES AND FABRICATION METHODS THEREOF,” U.S.Application No. 16,541,141, filed on even date, entitled“THREE-DIMENSIONAL MEMORY DEVICES AND FABRICATION METHODS THEREOF,” U.S.application Ser. No. 16/541,142, filed on even date, entitled“THREE-DIMENSIONAL MEMORY DEVICES AND FABRICATION METHODS THEREOF,” andU.S. application Ser. No. 16/541,144, filed on even date, entitled“THREE-DIMENSIONAL MEMORY DEVICES AND FABRICATION METHODS THEREOF,” allof which is hereby incorporated by reference in its entirety.

BACKGROUND

Embodiments of the present disclosure relate to three-dimensional (3D)memory devices and fabrication methods thereof.

Planar memory cells are scaled to smaller sizes by improving processtechnology, circuit design, programming algorithm, and fabricationprocess. However, as feature sizes of the memory cells approach a lowerlimit, planar process and fabrication techniques become challenging andcostly. As a result, memory density for planar memory cells approachesan upper limit.

A 3D memory architecture can address the density limitation in planarmemory cells. The 3D memory architecture includes a memory array andperipheral devices for controlling signals to and from the memory array.

SUMMARY

Embodiments of 3D memory devices and the fabrication methods tofabricate the 3D memory devices are disclosed herein.

In one example, a method for forming a 3D memory device includes thefollowing operations. First, an initial channel hole is formed in astack structure of a plurality first layers and a plurality of secondlayers alternatingly arranged over a substrate. An offset is formedbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole. Asemiconductor channel is formed by filling the channel hole with achannel-forming structure. The semiconductor channel may have a memorylayer including a plurality of first memory portions each surrounding abottom of a respective second layer and a plurality of second memoryportions each connecting adjacent first memory portions. Further, theplurality of second memory portions is removed to retain the pluralityof first memory portions. The plurality of first memory portions may bedisconnected from one another. In some embodiments, a plurality ofconductor layers are formed from the plurality of second layers and agate-to-gate dielectric layer is formed between the adjacent conductorlayers. The gate-to-gate dielectric layer includes at least onesub-layer of silicon oxynitride.

In another example, a method for forming a 3D memory device includes thefollowing operations. First, an initial channel hole is formed in astack structure of a plurality first layers and a plurality of secondlayers alternatingly arranged over a substrate. An offset is formedbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole. Asemiconductor channel is further formed by filling the channel hole witha channel-forming structure. The semiconductor channel may have a memorylayer including a plurality of first memory portions each surrounding abottom of a respective second layer and a plurality of second memoryportions each connecting adjacent first memory portions. Further, theplurality of second memory portions are removed to retain the pluralityof first memory portions. The plurality of first memory portions may bedisconnected from one another. A plurality of conductor layers areformed from the plurality of second layers and a gate-to-gate dielectricstructure that insulates each one of the plurality of conductor layersfrom one another is formed. The gate-to-gate dielectric structure mayinclude a plurality of composite layers separated by the plurality ofconductor layers. Each one of the plurality of composite layers mayinclude a stack of sub-layers having at least one sub-layer of siliconoxynitride.

In still another example, a 3D memory device includes a stack structurehaving a plurality of conductor layers insulated from one another by agate-to-gate dielectric structure. The gate-to-gate dielectric structuremay include at least a sub-layer of silicon oxynitride between adjacentconductor layers along a vertical direction perpendicular to a topsurface of the substrate. The 3D memory device may also include asemiconductor channel extending from a top surface of the stackstructure to the substrate. The semiconductor channel may include amemory layer that has a plurality of memory portions each surrounding abottom of a respective conductor layer and each being disconnected fromone another. The 3D memory device may further include a source structureextending from the top surface of the stack structure to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further serve to explain theprinciples of the present disclosure and to enable a person skilled inthe pertinent art to make and use the present disclosure.

FIGS. 1A-1F each illustrates a cross-sectional view of a portion of a 3Dmemory device, according to some embodiments of the present disclosure.

FIGS. 2A-2G illustrate structures of a 3D memory device at variousstages of an exemplary fabrication process, according to someembodiments of the present disclosure.

FIGS. 3A-3J illustrate structures of a 3D memory device at variousstages of another exemplary fabrication process, according to someembodiments of the present disclosure.

FIGS. 4A-4G illustrate structures of a 3D memory device at variousstages of another exemplary fabrication process, according to someembodiments of the present disclosure.

FIGS. 5A-5J illustrate structures of a 3D memory device at variousstages of another exemplary fabrication process, according to someembodiments of the present disclosure.

FIGS. 6A-6I illustrate structures of a 3D memory device at variousstages of another exemplary fabrication process, according to someembodiments of the present disclosure.

FIGS. 7A-7C each illustrates a cross-sectional view of a blocking layer,a memory layer, and a tunneling layer, according to some embodiments ofthe present disclosure.

FIGS. 8A and 8B each illustrates a cross-sectional view of agate-to-gate dielectric layer, according to some embodiments of thepresent disclosure.

FIG. 9A illustrates a flowchart of an exemplary method for forming asemiconductor channel in a stack structure, according to someembodiments of the present disclosure.

FIGS. 9B-9D each illustrates a flowchart of an exemplary method forforming a 3D memory device following the method of FIG. 9A, according tosome embodiments of the present disclosure.

FIG. 10 illustrates a flow chart of an exemplary method for forminganother 3D memory device, according to some embodiments of the presentdisclosure.

Embodiments of the present disclosure will be described with referenceto the accompanying drawings.

DETAILED DESCRIPTION

Although specific configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present disclosure. It will be apparent to aperson skilled in the pertinent art that the present disclosure can alsobe employed in a variety of other applications.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “some embodiments,” etc.,indicate that the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of aperson skilled in the pertinent art to effect such feature, structure orcharacteristic in connection with other embodiments whether or notexplicitly described.

In general, terminology may be understood at least in part from usage incontext. For example, the term “one or more” as used herein, dependingat least in part upon context, may be used to describe any feature,structure, or characteristic in a singular sense or may be used todescribe combinations of features, structures or characteristics in aplural sense. Similarly, terms, such as “a,” “an,” or “the,” again, maybe understood to convey a singular usage or to convey a plural usage,depending at least in part upon context. In addition, the term “basedon” may be understood as not necessarily intended to convey an exclusiveset of factors and may, instead, allow for existence of additionalfactors not necessarily expressly described, again, depending at leastin part on context.

It should be readily understood that the meaning of “on,” “above,” and“over” in the present disclosure should be interpreted in the broadestmanner such that “on” not only means “directly on” something but alsoincludes the meaning of “on” something with an intermediate feature or alayer therebetween, and that “above” or “over” not only means themeaning of “above” or “over” something but can also include the meaningit is “above” or “over” something with no intermediate feature or layertherebetween (i.e., directly on something).

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As used herein, the term “substrate” refers to a material onto whichsubsequent material layers are added. The substrate itself can bepatterned. Materials added on top of the substrate can be patterned orcan remain unpatterned. Furthermore, the substrate can include a widearray of semiconductor materials, such as silicon, germanium, galliumarsenide, indium phosphide, etc. Alternatively, the substrate can bemade from an electrically non-conductive material, such as a glass, aplastic, or a sapphire wafer.

As used herein, the term “layer” refers to a material portion includinga region with a thickness. A layer can extend over the entirety of anunderlying or overlying structure or may have an extent less than theextent of an underlying or overlying structure. Further, a layer can bea region of a homogeneous or inhomogeneous continuous structure that hasa thickness less than the thickness of the continuous structure. Forexample, a layer can be located between any pair of horizontal planesbetween, or at, a top surface and a bottom surface of the continuousstructure. A layer can extend laterally, vertically, and/or along atapered surface. A substrate can be a layer, can include one or morelayers therein, and/or can have one or more layer thereupon, thereabove,and/or therebelow. A layer can include multiple layers. For example, aninterconnect layer can include one or more conductor and contact layers(in which interconnect lines and/or via contacts are formed) and one ormore dielectric layers.

As used herein, the term “nominal/nominally” refers to a desired, ortarget, value of a characteristic or parameter for a component or aprocess operation, set during the design phase of a product or aprocess, together with a range of values above and/or below the desiredvalue. The range of values can be due to slight variations inmanufacturing processes or tolerances. As used herein, the term “about”indicates the value of a given quantity that can vary based on aparticular technology node associated with the subject semiconductordevice. Based on the particular technology node, the term “about” canindicate a value of a given quantity that varies within, for example,10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

As used herein, the term “3D memory device” refers to a semiconductordevice with vertically oriented strings of memory cell transistors(referred to herein as “memory strings,” such as NAND memory strings) ona laterally-oriented substrate so that the memory strings extend in thevertical direction with respect to the substrate. As used herein, theterm “vertical/vertically” means nominally perpendicular to the lateralsurface of a substrate.

As used herein, the terms “staircase,” “step,” and “level” can be usedinterchangeably. As used herein, a staircase structure refers to a setof surfaces that include at least two horizontal surfaces and at leasttwo vertical surfaces such that each horizontal surface is adjoined to afirst vertical surface that extends upward from a first edge of thehorizontal surface, and is adjoined to a second vertical surface thatextends downward from a second edge of the horizontal surface. A“staircase” refers to a vertical shift in the height of a set ofadjoined surfaces.

As used herein, the x-axis and the y-axis (perpendicular to the x-zplane) extend horizontally and form a horizontal plane. The horizontalplane is substantially parallel to the top surface of the substrate. Asused herein, the z-axis extends vertically, i.e., along a directionperpendicular to the horizontal plane. The terms “x-axis” and “y-axis”can be interchangeably used with “a horizontal direction,” the term “x-yplane” can be interchangeably used with “the horizontal plane,” and theterm “z-axis” can be interchangeably used with “the vertical direction.”

As 3D memory devices scale down for higher memory capacity, moreconductor layers, which function as gate electrodes of a 3D memorydevice, are stacked over a substrate within a designated space. Spacingbetween adjacent conductor layers along a vertical direction (i.e., thedirection perpendicular to a top surface of the substrate) is reduced,resulting in a thinner gate-to-gate dielectric layer between theadjacent conductor layers. Conventionally, the gate-to-gate dielectriclayer mainly includes silicon oxide (SiO_(x), e.g., SiO), of which theinsulation is largely affected by its thickness and film quality betweenthe adjacent conductor layers. Due to scaling, a thinner gate-to-gatedielectric layer, made of silicon oxide, can thus be susceptible togate-to-gate leakage or even breakdown. In addition, a reduced spacingbetween adjacent conductor layers can also cause increased charge loss.For example, due to smaller distance between adjacent memory cells,charges trapped in a memory cell is more likely to escape from thememory cell and travel along a memory layer (e.g., along its extendingdirection). As a result, data retention in the memory layer can beimpaired, and operations (e.g., read, write, and/or hold) on the memorycells may have reduced precision.

Various embodiments in accordance with the present disclosure providethe structures and fabrication methods of 3D memory devices, whichresolve the above-noted issues associated with thinner gate-to-gatedielectric layers. Embodiments of the present disclosure provide agate-to-gate dielectric layer having at least one composite layerbetween adjacent conductor layers. The composite layer includes at leastone sub-layer of silicon oxynitride (SiO_(x)N_(y), e.g., SiON). As ahigh-k dielectric material, silicon oxynitride can provide betterelectric insulation between adjacent conductor layers. The gate-to-gatedielectric layer, even with a smaller thickness between adjacentconductor layers, can reduce the susceptibility to leakage and coupling.In some embodiments, the gate-to-gate dielectric layer includes at leastan airgap between the adjacent conductor layers. In some embodiments,the gate-to-gate dielectric layer includes a pair of composite layerseach on a different one of the adjacent conductor layers, and an airgapbetween the two composite layers. In some embodiments, the gate-to-gatedielectric layer includes a composite layer filling up the space betweenadjacent conductor layers without any airgap in between. The compositelayer can include at least a sub-layer of silicon oxynitride. In someembodiments, the composite layer includes a plurality of sub-layers,which has at least one sub-layer of silicon oxynitride, each sandwichedby sub-layers of silicon oxide and/or silicon nitride. For example, thecomposite layer can include a plurality of alternatingly arrangedsub-layers of silicon oxynitride and silicon oxide.

Also, to reduce charge loss in 3D memory devices, in some embodiments,the memory layer in the semiconductor channel can have a “bent”structure or a “cut-off” structure to create a barrier between adjacentmemory cells (e.g., conductor layers) for the charges. In a “bent”structure, the memory layer has a plurality of first memory portions anda plurality second memory portions. Each first memory portion partiallysurrounds a respective conductor layer, and each second memory portionconnects adjacent first memory portions. The first memory portionincludes a vertical portion (e.g., extending vertically) and a pair oflateral portions (e.g., extending laterally), connected together topartially surround a bottom of the respective conductor layer. The firstmemory portions and the second memory portions may thus extend in astaggered manner along the vertical direction, creating a barrier forthe charges trapped in memory cells (e.g., first memory portions) alongthe vertical direction. This structure of the memory layer can reducecharge loss along the vertical direction. In a “cut-off” structure,different from the “bent” structure, the second memory portions betweenadjacent conductor layers are removed so the first memory portions aredisconnected from one another. This structure of the memory layer canenhance the barrier for the charges between adjacent memory cells.

FIGS. 1A-1E illustrate cross-sectional views of 3D memory devices eachhaving a gate-to-gate dielectric layer, according to the presentdisclosure. Specifically, FIG. 1A illustrates a memory device 101 havinga memory layer with a “cut-off” structure and a gate-to-gate dielectriclayer with an airgap between adjacent conductor layers. FIG. 1Billustrates a memory device 102 having a memory layer with a “cut-off”structure and a gate-to-gate dielectric layer without an airgap betweenadjacent conductor layers. FIG. 1C illustrates a memory device 103having a memory layer with a “bent” structure and a gate-to-gatedielectric layer with an airgap between adjacent conductor layers. FIG.1D illustrates a memory device 104 having a memory layer with a “bent”structure and a gate-to-gate dielectric layer without an airgap betweenadjacent conductor layers. FIG. 1E illustrates a memory device 105having a memory layer without a “bent” structure or a “cut-off”structure and a gate-to-gate dielectric layer with an airgap betweenadjacent conductor layers. FIG. 1F illustrates a memory device 106having a memory layer with a “bent” structure and a gate-to-gatedielectric layer with a pair of composite layers sandwiching adielectric layer of a different material. For ease of description, sameor similar parts in FIGS. 1A-1F are depicted using the same referencenumbers.

Embodiments of the present disclosure provide different types of memorydevices configured for reducing the leakage and coupling betweenconductor layers and preventing trapped charges to travel in undesireddirections. As examples, memory devices, having a semiconductor channelwith a “cut-off” structure and a gate-to-gate dielectric layer with atleast a sub-layer of a high-k dielectric material (e.g., siliconoxynitride) and an airgap, may be embodied by memory device 101. Memorydevices memory devices, having a semiconductor channel with a “bent”structure and a gate-to-gate dielectric layer with at least a sub-layerof a high-k dielectric material (e.g., silicon oxynitride), may beembodied by memory devices 103, 104, and 106. Memory devices, formed bya “gate first” fabrication process and having a gate-to-gate dielectriclayer with at least a sub-layer of a high-k dielectric material (e.g.,silicon oxynitride) and an airgap, may be embodied by memory devices101, 103, and 105. Memory devices, formed by a “gate first” fabricationprocess, having a semiconductor channel with a “bent” structure and agate-to-gate dielectric layer with at least a sub-layer of a high-kdielectric material (e.g., silicon oxynitride) and an airgap, may beembodied by memory device 103. Memory devices, having a semiconductorchannel with a “cut-off” structure and a gate-to-gate dielectric layerwith at least a sub-layer of a high-k dielectric material (e.g., siliconoxynitride), may be embodied by memory devices 101 and 102. Structuresand fabrication processes of the memory devices are described in detailas follows.

As shown in FIG. 1A, memory device 101 includes a substrate 10, aplurality of conductor layers 18 stacking over substrate 10, and aplurality of gate-to-gate dielectric layers 17 each between andinsulating adjacent conductor layers 18. Conductor layers 18, substrate10, and gate-to-gate dielectric layers 17 may form a stack structure.Memory device 101 may include a plurality of semiconductor channels 14each extending vertically (e.g., along a direction perpendicular to atop surface of substrate 10 or the y-direction) through the stackstructure into substrate 10. Memory device 101 may also include aplurality of source structures extending through the stack structure andinto substrate 10. Each source structure may include a doped region 16in substrate 10, an insulating structure 120 extending through the stackstructure, and a source contact 121 extending in insulating structure120 and contacting doped region 16. Source contact 121 may beelectrically connected to semiconductor channel 14 through doped region16 and substrate 10.

Substrate 10 can include silicon (e.g., single crystalline silicon),silicon germanium (SiGe), gallium arsenide (GaAs), germanium (Ge),silicon on insulator (SOD, and/or any other suitable materials. In someembodiments, substrate 10 includes silicon.

Conductor layers 18 can include conductive materials including, but notlimited to, tungsten (W), cobalt (Co), copper (Cu), aluminum (Al),polycrystalline silicon (polysilicon), doped silicon, silicides, or anycombination thereof.

Gate-to-gate dielectric layer 17 may include one or more compositelayers and at least an airgap between adjacent conductor layers 18. Inthe present disclosure, a plurality of gate-to-gate dielectric layers 17for insulating a plurality of conductor layers 18 in the stack structure(e.g., all the conductor layers 18 from top to bottom of the stackstructure) may be referred to as a gate-to-gate dielectric structure. Insome embodiments, gate-to-gate dielectric layer 17 includes a pair ofcomposite layers 17-1 and 17-2 and an airgap 173 between compositelayers 17-1 and 17-2. In some embodiments, composite layers 17-1 and17-2 may be formed in the space between adjacent conductor layers 18 andmay be on the opposing surfaces of adjacent conductor layers 18. In someembodiments, a thickness of a composite layer, e.g., 17-1 or 17-2, maybe less than about 5 nm, such as less than 5 nm (e.g., 0.5 nm, 1 nm, 1.5nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, any range bounded by thelower end by any of these values, or in any range defined by any two ofthese values). In some embodiments, a thickness of airgap 173 may bedependent on the thicknesses of composite layers 17-1 and 17-2, and thespacing between adjacent conductor layers 18.

Gate-to-gate dielectric layer 17 may include at least one sub-layer of ahigh-k dielectric material such as silicon oxynitride. In someembodiments, depending on the material of conductor layers 18, thehigh-k dielectric material may also include material other than siliconoxynitride. In some embodiments, each composite layer, e.g., 17-1 and17-2, may include a sub-layer of silicon oxynitride. Gate-to-gatedielectric layer 17 may also include sub-layers of other materials. Insome embodiments, each composite layer, e.g., 17-1 and 17-2, may includeat least a sub-layer of silicon oxide and/or silicon nitride. In someembodiments, each composite layer, e.g., 17-1 and 17-2, may include aplurality of sub-layers, having at least one sub-layer of siliconoxynitride, at least one sub-layer of silicon oxide, and at least onesub-layer of silicon nitride. In some embodiments, each composite layer,e.g., 17-1 and 17-2, may have a stack of sub-layers arranged asO/ON/O/ON/O, where “O” stands for silicon oxide and “ON” stands forsilicon oxynitride. In some embodiments, each composite layer, e.g.,17-1 and 17-2, may have a stack of sub-layers arranged asO/ON/O/N/O/ON/O. In some embodiments, along the vertical direction,conductor layer 18 and the composite layers formed on conductor layer 18(e.g., on the upper and lower surfaces of conductor layer 18) arelocated in the space defined between ends of vertical portion 132-1. Insome embodiments, a total thickness of conductor layer 18 and therespective composite layers is less than a distance between the ends ofvertical portion 132-1. In some embodiments, an end of lateral portion132-2 facing away from the respective vertical portion is exposed by arespective gate-to-gate dielectric layer 17. For example, the end may beexposed by airgap 173 of the respective gate-to-gate dielectric layer17. In some embodiments, a composite layer, similar to or the same as17-1 or 17-2, may be formed on the top surface of substrate 10.

FIG. 8A illustrates an exemplary structure of gate-to-gate dielectriclayer 17. As shown in FIG. 8A, x81 represents a sub-layer of siliconoxide, x82 represents a sub-layer of silicon oxynitride, and x83represents an airgap. Sub-layers x81, x82, and x81, on one of theadjacent conductor layers 18, may form a composite layer x8-1, andsub-layers x81, x82, and x81, on the other one of the adjacent conductorlayers 18, may form another composite layer x8-2. Composite layers x8-1,x8-2, and airgap x83 may form a gate-to-gate dielectric layer 17. Itshould be noted that the number of sub-layers in a composite layershould not be limited by the embodiments of the present disclosure. Insome embodiments, a thickness of each of composite layers x81 and x82 isless than about 5 nm, such as less than 5 nm (e.g., 0.5 nm, 1 nm, 1.5nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, any range bounded by thelower end by any of these values, or in any range defined by any two ofthese values).

Semiconductor channel 14 may include a blocking layer 131, a memorylayer 132, a tunneling layer 133, a semiconductor layer 134, and adielectric core 19, arranged along a radial direction from the sidewalltowards the center of semiconductor channel 14. Blocking layer 131 mayinclude a plurality of blocking portions, each under a bottom of arespective conductor layer 18 and disconnected from one another. Memorylayer 132 may include a plurality of memory portions, each under thebottom of the respective conductor layer 18 and partially surrounds therespective conductor layer 18. Each memory portion may be disconnectedfrom one another. A memory portion may include a vertical portion 132-1(e.g., extending along the vertical direction or the y-direction) and atleast one lateral portion 132-2 (e.g., extending along the lateraldirection or the x-direction) connected to vertical portion 132-1. Insome embodiments, a memory portion includes a vertical portion 132-1 anda pair of lateral portions 132-2 (e.g., each connected to a differentend of vertical portion 132-1). One end of lateral portion 132-2 may beconnected to the respective vertical portion 132-1, and the other end oflateral portion 132-2 may be facing away from the respective verticalportion 132-1 (e.g., being exposed by airgap 173). The memory portionmay be under and partially surrounding the respective block portion.Tunneling layer 133, exposed by airgaps 173, may be under and partiallysurrounding the respective memory portion.

Blocking layer 131 can reduce or prevent charges from escaping intoconductor layers 18. Blocking layer 131 can include a single-layeredstructure or a multiple-layered structure. For example, blocking layer131 can include a first blocking layer and a second blocking layer. Thefirst blocking layer can be formed over the sidewall of a channel hole,and the second blocking layer may be formed over the first blockinglayer. The first blocking layer can include a dielectric material (e.g.,a dielectric metal oxide.) For example, the first blocking layer caninclude a dielectric metal oxide having a sufficiently high dielectricconstant (e.g., greater than 7.9.) Examples of the first blocking layerinclude AlO, hafnium oxide (HfO₂), lanthanum oxide (LaO₂), yttrium oxide(Y₂O₃), tantalum oxide (Ta₂O₅), silicates thereof, nitrogen-dopedcompounds thereof, and/or alloys thereof. The second blocking layer caninclude a dielectric material that is different from the first blockinglayer. For example, the second blocking layer can include silicon oxide,silicon oxynitride, and/or silicon nitride. FIG. 7A illustrates anexemplary blocking layer x31, which is the same as or similar toblocking layer 131. As shown in FIG. 7A, blocking layer x31 includes afirst blocking layer x31 a and a second blocking layer x31 b. Firstblocking layer x31 a may include a high-k dielectric layer such as AlO.Second blocking layer x31 b may include a plurality of dielectric layersstacking laterally. For example, second blocking layer x31 b may includea pair of first dielectric layers x31 c and a second dielectric layerx31 d, where second dielectric layer x31 d is sandwiched by firstdielectric layers x31 c. In some embodiments, first dielectric layer x31c includes a silicon oxide, and second dielectric layer x31 d includessilicon oxynitride.

Memory layer 132 can include a charge-trapping material and can beformed over blocking layer 131. Memory layer 132 can include asingle-layered structure or a multiple-layered structure. For example,memory layer 132 can include conductive materials and/or semiconductorsuch as tungsten, molybdenum, tantalum, titanium, platinum, ruthenium,alloys thereof, nanoparticles thereof, silicides thereof, and/orpolycrystalline or amorphous semiconductor materials (e.g., polysiliconand amorphous silicon). Memory layer 132 can also include one or moreinsulating materials such as SiN and/or SiON. FIG. 7B illustrates anexemplary memory layer x32, which is the same as or similar to memorylayer 132. As shown in FIG. 7B, memory layer x32 may include a pluralityof alternatingly arranged first memory sub-layers x32 a and secondmemory sub-layers x32 b. In some embodiments, first memory sub-layer x32a includes silicon nitride, and second memory sub-layer x32 b includessilicon oxynitride.

Tunneling layer 133 can include a dielectric material through whichtunneling can occur under a suitable bias. Tunneling layer 133 can beformed over memory layer 132 and can include a single-layered structureor a multiple-layered structure. Tunneling layer 133 may include SiO,SiN, SiON, dielectric metal oxides, dielectric metal oxynitride,dielectric metal silicates, and/or alloys thereof. FIG. 7C illustratesan exemplary tunneling layer x33, which is the same as or similar totunneling layer 133. As shown in FIG. 7C, tunneling layer x33 mayinclude a plurality of first tunneling sub-layers x33 a and a secondtunneling sub-layer x33 b. In some embodiments, second tunnelingsub-layer x33 b may be sandwiched by a pair of first tunnelingsub-layers x33 a. In some embodiments, first tunneling sub-layer x33 aincludes silicon oxide, and second tunneling sub-layer x33 b includes aplurality of layers of silicon oxynitride.

Semiconductor layer 134 can facilitate the transport of charges and canbe formed over tunneling layer 133. Semiconductor layer 134 can includeone or more semiconductor materials such as a one-element semiconductormaterial, an III-V compound semiconductor material, an II-VI compoundsemiconductor material, and/or an organic semiconductor material. Insome embodiments, semiconductor layer 134 includes a poly-silicon layer.

Dielectric core 19 can include a suitable dielectric material and canfill up the space surrounded by semiconductor layer 134. In someembodiments, dielectric core 19 includes silicon oxide (e.g., siliconoxide of sufficiently high purity).

Doped region 16 can be formed in substrate 10, contacting source contact121. Source contact 121 may be insulated from conductor layers 18 byinsulating structure 120. Source contact 121 may include any suitableconductive material that can be used as the source electrode, and dopedregion 16 may include a suitable doped (e.g., P-type or N-type)semiconductor region formed in substrate 10 and is opposite of thepolarity of substrate 10. In some embodiments, source contact 121includes one or more of doped poly-silicon, copper, aluminum, cobalt,doped silicon, silicides, and tungsten. In some embodiments, dopedregion 16 includes doped silicon. In some embodiments, insulatingstructure 120 includes silicon oxide.

FIG. 1B illustrates a cross-section view of memory device 102, accordingto some embodiments. Different from memory device 101, gate-to-gatedielectric layer 17 has no airgap between adjacent conductor layers 18and fills up the space between adjacent conductor layers 18 with acomposite layer. In some embodiments, insulating structure 120 insulatessource contact 121 from conductor layers 18 and gate-to-gate dielectriclayers 17. In some embodiments, the ends of lateral portions 132-2,exposed portions of blocking layer 131, and exposed portions oftunneling layer 133, are covered by gate-to-gate dielectric layer 17. Insome embodiments, a composite layer fills up the space between substrate10 and the conductor layer 18 closest to substrate 10. FIG. 8Billustrates an exemplary structure of the composite layer. As shown inFIG. 8B, the composite layer may include a plurality of sub-layers,where at least one of the sub-layers include silicon oxynitride. In someembodiments, at least one of the sub-layers include silicon oxynitrideand at least one of the sub-layers include silicon oxide. In someembodiments, at least one of the sub-layers include silicon oxynitride,at least one of the sub-layers include silicon oxide, and at least oneof the sub-layers include silicon nitride. In some embodiments, x81represents silicon oxide and x82 represents silicon oxynitride, and thecomposite layer include a plurality of alternatingly arranged sub-layersof silicon oxynitride and silicon oxide. In some embodiments, the numberof sub-layers of each material and the thickness of each sub-layer maybe associated with, e.g., the total thickness of the composite layer(e.g., the spacing between adjacent conductor layers 18) and/or thefabrication process, and should not be limited by the embodiments of thepresent disclosure.

FIG. 1C illustrates a cross-section view of memory device 103, accordingto some embodiments. Different from memory device 101, blocking layer131 and memory layer 132 extend consistently along the horizontaldirection and the vertical direction. Memory layer 132 may include afirst memory portion 132 a under and partially surrounding a bottom ofthe respective conductor layer 18 and composite layers on respectiveconductor layer 18, and a second memory portion 132 b connected toadjacent first memory portions 132 a. As shown in FIG. 1C, blockinglayer 131 may be over memory layer 132, and may accordingly be under andpartially surrounding the bottom of the respective conductor layer 18and composite layers on respective conductor layer 18. The lateralportions of blocking layer 131 may have contact with composite layerslaterally. First memory portion 132 a may include a vertical portion 132a-1 and at least one lateral portion 132 a-2. In some embodiments, thefirst portion may include vertical portion 132 a-1 and a pair of lateralportions 132 a-2. In some embodiments, second memory portion 132 bextends vertically. As shown in FIG. 1C, second memory portions 132 band vertical portions 132 a-1 of memory layer 132 may be staggered alongthe vertical direction. In some embodiments, a thickness of a compositelayer, e.g., 17-1 or 17-2, may be less than about 5 nm, such as lessthan 5 nm (e.g., 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm,4.5 nm, any range bounded by the lower end by any of these values, or inany range defined by any two of these values). A detailed description ofgate-to-gate dielectric layer 17 and composite layers 17-1 and 17-2 maybe referred to the description of gate-to-gate dielectric layer 17 andcomposite layers 17-1 and 17-2 in memory device 101, and is not repeatedherein.

FIG. 1D illustrates a cross-sectional view of memory device 104,according to some embodiments. Different from memory device 103,gate-to-gate dielectric layer 17 has no airgap between adjacentconductor layers 18 and fills up the space between adjacent conductorlayers 18 with a composite layer. In some embodiments, a composite layerfills up the space between substrate 10 and the conductor layer 18closest to substrate 10. A detailed description of structures andmaterials of gate-to-gate dielectric layer 17 and the composite layermay be referred to the description of gate-to-gate dielectric layer 17and the composite layer in memory device 102, and is not repeatedherein.

FIG. 1E illustrates a cross-sectional view of memory device 105,according to some embodiments. Different from memory devices 101 and103, memory device 105 includes a semiconductor channel 14 in whichblocking layer 131, memory layer 132, tunneling layer 133, andsemiconductor layer 134 each extends continuously along the verticaldirection. In some embodiments, a thickness of a composite layer, e.g.,17-1 or 17-2, may be less than about 5 nm, such as less than 5 nm (e.g.,0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, anyrange bounded by the lower end by any of these values, or in any rangedefined by any two of these values). A detailed description ofgate-to-gate dielectric layer 17 may be referred to the description ofmemory device 101 and is not repeated herein.

FIG. 1F illustrates a cross-sectional view of memory device 106,according to some embodiments. Different from memory device 104, memorydevice 106 includes a dielectric layer 170 sandwiched by a pair ofcomposite layers 17-1 and 17-2, where dielectric layer 170 includes amaterial that is different from the materials of composite layers 17-1and 17-2. In some embodiments, dielectric layer 170 includes siliconnitride. Optionally, an adhesive layer 124, including titanium and/ortitanium oxide, is formed between conductor layer 18 and gate-to-gatedielectric layer 17. In some embodiments, a thickness of a compositelayer, e.g., 17-1 or 17-2, may be less than about 5 nm, such as lessthan 5 nm (e.g., 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm,4.5 nm, any range bounded by the lower end by any of these values, or inany range defined by any two of these values). A detailed description ofstructures and materials of composite layers 17-1 and 17-2 may bereferred to the description of composite layers 17-1 and 17-2 of memorydevice 101, and is not repeated herein.

FIGS. 2A-2G illustrate a method for forming a stack structure withsemiconductor channels with “bent” structures, according to someembodiments. Structure 200 depicted in FIG. 2G can be used as the basestructure to form memory devices 101-104. FIG. 9A illustrates theflowchart of fabrication process 900 depicted in FIGS. 2A-2G.

Referring to FIG. 9A, at the beginning of the fabrication process, aninitial channel hole is formed in a stack structure that has a pluralityof alternatingly arranged first layers and second layers over asubstrate (Operation 902). FIGS. 2A and 2B illustrate correspondingstructures.

As shown in FIG. 2A, a stack structure 21 having a plurality ofalternatingly arranged first layers 211 and second layers 212 is formedover a substrate 20. The material of substrate 20 may be referred to thedescription of substrate 10 and is not repeated herein. In someembodiments, substrate 20 includes silicon (N-type silicon).

Stack structure 21 can provide the fabrication base for the formation ofa 3D memory device. Memory strings (e.g., NAND memory strings) thatinclude semiconductor channels and related structures/parts can besubsequently formed in stack structure 21. In some embodiments, stackstructure 21 includes a plurality of first layer 211/second layer 212pairs stacked vertically over substrate 20, forming a staircasestructure. Each first layer 211/second layer 212 pair can include onefirst layer 211 and one second layer 212, and can form astaircase/level. That is, stack structure 21 can include interleavedfirst layers 211 and second layers 212 stacked along the verticaldirection. The number of first layer 211/second layer 212 pairs in stackstructure 21 (e.g., 32, 64, 96, or 128) can set the number of memorycells in the 3D memory device.

First layers 211 can each have the same thickness or have differentthicknesses. Similarly, second layers 212 can each have the samethickness or have different thicknesses. Second layers 212 can includeany suitable materials that are different from the material of firstlayers 211 so that an etchant (e.g., used in the subsequent fabricationprocess to remove first layers 211) can have a higher etch rate on firstlayers 211 over second layers 212. That is, the etchant can selectivelyetch first layers 211 over second layers 212. In some embodiments, firstlayers 211 can include a sacrificial material and second layers 212 caninclude a conductor material. In some embodiments, first layers 211 caninclude a sacrificial material and second layers 212 can include anothersacrificial layer. The specific choices of materials of first layers 211and second layers 212 should be determined by the fabrication process(e.g., the gate-first fabrication process or the gate-last fabricationprocess) and will be explained in detail as follows.

Stack structure 21 can be formed by, e.g., repetitively etching adielectric stack of a plurality of first material layer/second materiallayer pairs vertically and laterally. The etching of the first materiallayer/second material layer pairs can include repetitivelyetching/trimming an etch mask (e.g., a photoresist layer) over thedielectric stack to expose the portion of first material layer/secondmaterial layer pair to be etched, and etching/removing the exposedportion using a suitable etching process. The etching of the etch maskand the insulating material layer/sacrificial material layer pairs canbe performed using any suitable etching processes such as wet etchand/or dry etch. In some embodiments, the etching includes dry etch,e.g., inductively coupled plasma etching (ICP) and/or reactive-ion etch(RIE).

An initial channel hole 22 can be formed in stack structure 21. In someembodiments, initial channel hole 22 extends from a top surface of stackstructure 21 to substrate 20. In some embodiments, a bottom portion ofinitial channel hole 22 exposes substrate 20. Initial channel hole 22can be formed by any suitable fabrication process. For example, apatterned photoresist layer can be formed over stack structure 21. Thepatterned photoresist layer can expose a portion of stack structure 21for forming initial channel hole 22. A suitable etching process can beperformed to remove the portion of stack structure 21 until substrate 20is exposed. The etching process can include a dry etching process.

Referring back to FIG. 9A, after initial channel holes are formed, achannel hole is formed by removing a portion of each first layer on asidewall of the initial channel hole to form an offset between a sidesurface of a second layer and side surfaces of adjacent first layers(Operation 904). FIG. 2C illustrates a corresponding structure.

As shown in FIG. 2C, a portion of each first layer 211 on the sidewallof initial channel hole 22 can be removed to form channel hole 222. Forease of description, the surface of first layer 211 (or second layer212) facing initial channel hole 22 or channel hole 222 is referred toas a side surface of first layer 211 (or second layer 212). In someembodiments, an offset 224 can be formed on the side surface of firstlayer 211. The dimension or thickness of the removed portion (e.g.,along the later direction or the x-direction) of first layer 211 can beany suitable value that allows an offset to be formed between the sidesurface of second layer 212 and first layer 211. In some embodiments,the side surfaces of second layers 212 form protrusions along thesidewall of channel hole 222. Any suitable selective etching process(e.g., a recess etch) can be performed to form offsets 224. In someembodiments, the selective etching process has a high etchingselectivity on first layers 211 over second layers 212, causing littleor no damage on second layers 212. A wet etch and/or a dry etch can beperformed as the selective etching process. In some embodiment, an RIEis performed as the selective etching process.

Referring to FIG. 9A, after the formation of the channel hole, achannel-forming structure is formed to fill up the channel hole, and asemiconductor channel is formed (Operation 906). FIGS. 2D-2F illustratescorresponding structures.

As shown in FIG. 2D-2F, a semiconductor channel 24 can be formed byfilling channel hole 222 with a channel-forming structure. The channelforming structure may include a blocking layer 231 deposited along thesidewall of channel hole 222, a memory layer 232 over the blockinglayer, a tunneling layer 233 over the blocking layer, a semiconductorlayer 234 over the tunneling layer, and a dielectric core 29 filling upthe rest of channel hole 222. Each of these layers may be respectivelythe same as or similar to blocking layer 131, memory layer 132,tunneling layer 133, semiconductor layer 134, and dielectric core 19illustrated in FIG. 1A. A detailed description of the materials of theselayers is thus not repeated herein.

As shown in FIG. 2D, in some embodiments, a blocking material layer, amemory material layer, and a tunneling material layer, are sequentiallydeposited in channel hole 222 along a radial direction from the sidewalltowards the center of channel hole 222. The materials of the blockingmaterial layer, the memory material layer, and the tunneling materiallayer can be referred to the description of blocking layer 131, memorylayer 132, and tunneling layer 133, and are not repeated herein. Theblocking material layer can be formed by a suitable deposition methodsuch as chemical vapor deposition (CVD), atomic layer deposition (ALD),pulsed laser deposition (PLD), low pressure CVD (LPCVD), and/or liquidsource misted chemical deposition. The memory material layer can beformed by any suitable deposition method such as CVD, ALD, and physicalvapor deposition (PVD). The tunneling material layer can be formed by asuitable deposition method such as CVD, ALD, and/or PVD. A recessetching process, such as dry etch, can be performed to remove portionsof the blocking material layer, the memory material layer, and thetunneling material layer at the bottom of channel hole 222 to exposesubstrate 20. Blocking layer 231, memory layer 232, and tunneling layer233 can then be formed accordingly.

As shown in FIGS. 2E and 2F, a semiconductor layer 234 is deposited overtunneling layer 233 and substrate 20, and a dielectric core 29 isdeposited over semiconductor layer 234 to fill up the rest of the spacein channel hole 222, forming semiconductor channel 24. Semiconductorlayer 234 can be formed by any suitable deposition method such as LPCVD,ALD, and/or metal-organic chemical vapor deposition (MOCVD). In someembodiments, dielectric core 29 includes SiO (e.g., SiO of sufficientlyhigh purity) and can be formed by any suitable deposition method such asCVD, LPCVD, ALD, and/or PVD.

Referring back to FIG. 9A, after the formation of the semiconductorchannel, a first initial slit opening is formed in the stack structure(Operation 908). FIG. 2G illustrates a corresponding structure 200.

As shown in FIG. 2G, a first initial slit opening 25 is formed to extendthrough the stack structure and expose substrate 20. A suitable etchingprocess, e.g., a dry etching process, can be performed to form firstinitial slit opening 25.

FIGS. 3A-3J illustrate a “gate first” method to form memory devices 103and 104 based on structure 200, according to some embodiments.Specifically, FIGS. 3A, 3C, 3E, 3G, and 3I illustrate the fabricationprocess to form memory device 103 based on structure 200, and FIGS. 3B,3D, 3F, 3H, and 3J illustrate the fabrication process to form memorydevice 104 based on structure 200. In the “gate first” method, firstlayers 211 include a sacrificial material and second layers 212 includea conductor material for subsequently forming conductor layers 18. Insome embodiments, second layers 212 include polysilicon. FIG. 9Billustrates the flowchart of fabrication process 920 depicted in FIGS.3A-3J to form memory devices 103 and 104.

As shown in FIG. 9B, at the beginning of the fabrication process, theplurality of first layers are removed (Operation 922) and a gate-to-gatedielectric layer is formed between adjacent conductor layers (Operation924). A second initial slit opening is formed from the first initialslit opening. FIGS. 3A and 3B respectively illustrate a correspondingstructure. In some embodiments, an isotropic etching process (e.g., wetetch) is performed to remove first layers 211 and expose blocking layer231 and substrate 20. A plurality of lateral recesses can be formed fromthe removal of first layers 211.

As shown in FIG. 3A, an oxidation reaction and/or a nitriding reactionmay be performed to form a composite layer from a portion of secondlayer 212 that reacts with the reactants. The unreacted portion ofsecond layer 212 may form a conductor layer 38 that can function as agate electrode of memory device 103. The reacted portion of second layer212 may form a composite layer 37-1 or 37-2 (e.g., similar to or thesame as 17-1 or 17-2) covering conductor layer 38. The composite layermay be formed from a top portion/upper surface of second layer 212 andfrom a bottom portion/lower surface of second layer 212. An airgap 373may be formed between composite layers 37-1 and 37-2 on adjacentconductor layers 38. In some embodiments, a pair of composite layers(e.g., 37-1 and 37-2) facing each other and on adjacent conductor layers38 and airgap 373 in between may form a gate-to-gate dielectric layer37, similar to or the same as gate-to-gate dielectric layer 17illustrated in FIGS. 1A and 1C. In some embodiments, the composite layer(e.g., 37-1 or 37-2) may also be formed on the side surface of secondlayers 212 (e.g., the sidewall of first initial slit opening 25),forming a second initial slit opening 35A from first initial slitopening 25.

In some embodiments, a plurality of gate-to-gate dielectric layers 37are formed by oxidizing and/or nitriding second layers 212 through firstinitial slit opening 25 and the lateral recesses. In some embodiments,to form plurality of gate-to-gate dielectric layers 37, oxygen diffusionconcentration and/or nitrogen diffusion concentration is controlled,such that each gate-to-gate dielectric layer 37 includes at least onesub-layer of silicon oxynitride. In some embodiments, each compositelayer (e.g., 37-1 or 37-2) includes at least a sub-layer of siliconoxynitride. In some embodiments, oxygen and/or nitrogen diffusionconcentration are controlled, so each of the plurality of gate-to-gatedielectric layers 37 can have the structures described in FIG. 1A. Forexample, each gate-to-gate dielectric layer 37 includes a pair ofcomposite layers (e.g., 37-1 and 37-2), each including a plurality ofalternatingly arranged sub-layers of silicon oxynitride and siliconoxide. The specific structure of each composite layer should not belimited by the embodiments of the present disclosure. In someembodiments, a composite layer may be formed over substrate 20 from theoxidation and/or nitridation reaction.

Different from the process to form gate-to-gate dielectric layer 37 fromportions of second layers 212, as shown in FIG. 3B, gate-to-gatedielectric layer 37 can be formed by depositing a dielectric material tofill up the lateral recesses and performing an oxidizing reaction and/ora nitriding reaction to form the at least one sub-layer of siliconoxynitride in each gate-to-gate dielectric layer 37. The process can beperformed through the lateral recess and first initial slit opening 25.In some embodiments, a dielectric material, such as silicon oxide orsilicon nitride, may be deposited by a suitable deposition method, e.g.,CVD, ALD, and/or PVD, to fill up the lateral recesses. An oxidizingreaction and/or a nitriding reaction may be performed on the depositeddielectric material between adjacent second layers 212 to formgate-to-gate dielectric layer 37, which includes a composite layerhaving at least one sub-layer of silicon oxynitride. In someembodiments, each composite layer includes at least a sub-layer ofsilicon oxynitride. In some embodiments, oxygen and/or nitrogendiffusion concentration are controlled so each of the plurality ofgate-to-gate dielectric layers 37 can have the structures described inFIG. 1B. For example, each gate-to-gate dielectric layer 37 includes acomposite layer having a plurality of alternatingly arranged sub-layersof silicon oxynitride and silicon oxide. No airgap is formed betweenadjacent second layers 212. In some embodiments, gate-to-gate dielectriclayer 37 covers blocking layer 231. The specific structure of eachcomposite layer should not be limited by the embodiments of the presentdisclosure. In some embodiments, second layers 212 form conductor layers38. In some embodiments, an adhesion layer (not shown) may be formed onsecond layers 212 before the deposition of the dielectric material. Insome embodiments, the composite layer may also be formed on the sidesurface of second layers 212 (e.g., the sidewall of first initial slitopening 25), forming a second initial slit opening 35B from firstinitial slit opening 25. In some embodiments, a composite layer may beformed over substrate 20 from the oxidation and/or nitridation reaction.

Referring back to FIG. 9B, after the formation of gate-to-gatedielectric layers, a doped region may be formed in the substrate at abottom of the second initial slit opening (Operation 926). FIGS. 3C and3D illustrate corresponding structures.

As shown in FIGS. 3C and 3D, a doped region 36 may be formed insubstrate 20 at the bottom of the second initial slit opening (e.g., 35Ain FIGS. 3C and 35B in FIG. 3D). A suitable doping process, such as ionimplantation, can be performed to form doped region 36. In someembodiments, a portion of the composite layer at the bottom of thesecond initial slit opening (e.g., 35A and 35B) is removed to exposesubstrate 20 before the doping process. In some embodiments, the portionof the composite layer at the bottom of the second initial slit opening(e.g., 35A and 35B) is retained.

Referring back to FIG. 9B, after the formation of the doped region, aslit opening is formed from the second initial slit opening (Operation928). FIGS. 3E and 3F illustrate corresponding structures.

As shown in FIGS. 3E and 3F, a slit opening (e.g., 350A in FIG. 3E and350B in FIG. 3F) is formed from respective second initial slit opening(e.g., 35A in FIG. 3C and 35B in FIG. 3D). In some embodiments, a recessetch is performed to remove any excess materials from the side surfacesof conductor layers 38, forming slit opening 350A/350B. In someembodiments, excess material (e.g., the material of a composite layer)over substrate 20 at the bottom of second initial slit opening 35A/35Bcan also be etched and removed. The sidewall of slit opening 350A/350Bmay expose conductor layers 38. In some embodiments, the sidewall ofslit opening 350A exposes airgaps 373. In some embodiments, the sidewallof slit opening 350A/350B also exposes gate-to-gate dielectric layers37.

Referring back to FIG. 9B, an insulating structure is formed in the slitopening (Operation 930). FIGS. 3G and 3H illustrate correspondingstructures.

As shown in FIGS. 3G and 3H, an insulating structure (e.g., 320A in FIG.3G and 320B in FIG. 3H) may be formed in respective slit structure(e.g., 350A in FIG. 3G and 350B in FIG. 3H). In some embodiments,insulating structure 320A/320B is formed over the sidewall of respectiveslit opening 350A/350B and exposes substrate 20 (e.g., or doped region36) at the bottom of respective slit opening 350A/350B. In someembodiments, insulating structure 320A/320B includes a dielectricmaterial, such as silicon oxide, and is deposited by a suitabledeposition process such as CVD, ALD, LPCVD, and/or PVD. In someembodiments, a recess etch (e.g., dry etch and/or wet etch) is performedto remove any excess material (e.g., material deposited during theformation of insulating structure 320A/320B) at the bottom of slitstructure 350A/350B to expose substrate 20 (e.g., or doped region 36).

Referring back to FIG. 9B, after the formation of insulating structure,a source contact is formed in the insulating structure (Operation 932).FIGS. 3I and 3J illustrate corresponding structures.

As shown in FIGS. 3I and 3J, a suitable conductive material can bedeposited in insulating structure 320A/320B to form a respective sourcecontact 321. Any suitable deposition method can be used to form sourcecontact 321. For example, source contact 321 can be formed by CVD, ALD,and/or PVD. In some embodiments, source contact 321 includes tungstenand is deposited by CVD. In some embodiments, source contact 321A, dopedregion 36, and respective insulating structure 320A/320B form a sourcestructure. A suitable planarization process (e.g., recess etch and/orchemical-mechanical polishing) can be performed to planarize the topsurface of the stack structure, e.g., planarizing the source structures,semiconductor channels 24, and/or gate-to-gate dielectric layers 37.

FIGS. 4A-4G illustrate a “gate first” method to form memory devices 101and 102 based on structure 200, according to some embodiments.Specifically, FIGS. 4A, 4B, 4D, and 4F illustrate the fabricationprocess to form memory device 101 based on structure 200, and FIGS. 4A,4C, 4E, and 4G illustrate the fabrication process to form memory device102 based on structure 200. In the “gate first” method, first layers 211include a sacrificial material and second layers 212 include a conductormaterial for subsequently forming conductor layers 18. In someembodiments, second layers 212 include polysilicon. FIG. 9C illustratesa flowchart 940 for fabrication processes depicted in FIGS. 4A-4G toform memory devices 101 and 102.

As shown in FIG. 9C, at the beginning of the fabrication process, theplurality of first layers are removed (Operation 942) and a memory layerhaving a memory portion under a bottom of each second layer is formed(Operation 944). The memory portions are disconnected from one another.FIG. 4A illustrates a corresponding structure. In some embodiments, anisotropic etching process (e.g., wet etch) is performed to remove thefirst layers (e.g., 211) to form a plurality of lateral recesses thatexpose the blocking layer (e.g., 231) and the substrate (e.g., 20).

As shown in FIG. 4A, a blocking layer 431 having a plurality of blockingportions, each under a bottom of a respective second layer 212 anddisconnected from each other, is formed. Also, a memory layer 432 havinga plurality of memory portions, each under a respective blockingportion, is formed. Each memory portion may include a vertical portion432-1 and at least one lateral portion 432-2 connected to verticalportion 432-1. In some embodiments, each memory portion includes a pairof lateral portions 432-2 being connected to a different end of therespective vertical portion 432-1. Each memory portion may surround therespective blocking portion under the bottom of the respective secondlayer 212 and may be disconnected from one another along the verticaldirection. A tunneling layer 433 under and partially surrounding memorylayer 432 is also formed and extend along the vertical directionconsistently. In some embodiments, tunneling layer 433 may be exposedbetween adjacent second layers 212.

A suitable etching process (e.g., a wet etch) may be performed onstructure 200 to remove portions of semiconductor channel 24 from firstinitial slit opening 25 and the lateral recesses. In some embodiments,at least second memory portions 232 b are removed to expose lateralportions 232 a-2 of first memory portions 232 a. First memory portions232 a may fully or partially be retained to form the memory portions.Depending on the etching process, lateral portions 232-2 may beover-etched, and the length of lateral portion 232 a-2 may vary alongthe lateral direction in different applications. In some embodiments,portions of blocking layer 231 and tunneling layer 233 may also beremoved during the etching process. Blocking portions, disconnected fromone another and over memory portions, may be formed. Semiconductorchannel 24, after the formation of memory portions, may form asemiconductor channel 44.

Referring back to FIG. 9C, a gate-to-gate dielectric layer is formedbetween adjacent conductor layers and a second initial slit opening isformed (Operation 946). Also, a doped region is formed in the substrateat the bottom of the second initial list opening (Operation 948). FIGS.4B and 4C respectively illustrate a corresponding structure.

FIG. 4B illustrates a gate-to-gate dielectric layer 47 with an airgap.As shown in FIG. 4B, a gate-to-gate dielectric layer 47, a conductorlayer 48, a second initial opening 45A, and a doped region 46 may beformed in the stack structure. In some embodiments, gate-to-gatedielectric layer 47 includes a pair of composite layers 47-1 and 47-2,and an airgap 473 between composite layers 47-1 and 47-2. Thefabrication process to form these structures may be referred to thefabrication process to form gate-to-gate dielectric layer 37, conductorlayer 38, second initial slit opening 35A, and doped region 36illustrated in FIGS. 3A and 3C, and is not repeated herein.

FIG. 4C illustrates a gate-to-gate dielectric layer 47 without anairgap. As shown in FIG. 4C, gate-to-gate dielectric layer 47, aconductor layer 48, a second initial opening 45B, and a doped region 46may be formed in the stack structure. In some embodiments, gate-to-gatedielectric layer 47 includes a composite layer filling up the spacebetween adjacent conductor layers 48. In some embodiments, gate-to-gatedielectric layer 47 covers the exposed portions of blocking layer 431,memory layer 432, and tunneling layer 433. The fabrication process toform these structures may be referred to the fabrication process to formgate-to-gate dielectric layer 37, conductor layer 38, second initialslit opening 35B, and doped region 36 illustrated in FIGS. 3B and 3D,and is not repeated herein.

Referring back to FIG. 9C, after the formation of the doped region andgate-to-gate dielectric layer, a slit opening is formed from the secondinitial slit opening (Operation 950) and an insulating structure isformed in the slit opening (Operation 952). FIGS. 4D and 4E respectivelyillustrate a corresponding structure.

As shown in FIGS. 4D and 4E, a slit opening (e.g., 450A in FIG. 4D and450B in FIG. 4E) and an insulating structure (e.g., 420A in FIG. 4D and420B in FIG. 4E) can be formed. The fabrication process to form slitopening 450A and insulating structure 420A may be referred to thefabrication process to form slit opening 350A and insulating structure320A in FIGS. 3E and 3G, and the fabrication process to form slitopening 450B and insulating structure 420B may be referred to thefabrication process to form slit opening 350B and insulating structure320B in FIGS. 3F and 3H. Details are not repeated herein.

Referring back to FIG. 9C, after the formation of the slit opening andthe insulating structure, a source contact is formed in the insulatingstructure (Operation 954). FIGS. 4F and 4G respectively illustrate acorresponding structure.

As shown in FIGS. 4F and 4G, a source contact 421 is formed inrespective insulating structure (e.g., 420A in FIG. 4F and 420B in FIG.4G), contacting the respective doped region 46. The fabrication processto form source contact 421 can be referred to the fabrication process toform source contact 321 illustrated in FIGS. 3I and 3J. Details are notrepeated herein.

FIGS. 5A-5D, 5E, and SI illustrate a “gate first” method to form memorydevice 105, which has an airgap in a gate-to-gate dielectric layer,according to some embodiments. FIGS. 5A-5D, 5F, and 5J illustrate a“gate first” method to form a memory device without an airgap in agate-to-gate dielectric layer, according to some embodiments. FIG. 10illustrates a flowchart 1000 for fabrication processes depicted in FIGS.5A-5J.

At the beginning of the fabrication process, a semiconductor channel isformed in a stack structure (Operation 1002). FIGS. 5A-5C illustratecorresponding structures.

As shown in FIGS. 5A-5C, a semiconductor channel 54 can be formed in astack structure 51 over a substrate 50. As shown in FIG. 5A, stackstructure 51 may include a plurality of alternatingly arranged firstlayers 511 and second layers 512 forming a plurality of staircases,where each first layer 511/second layer 512 form a staircase/level.First layers 511 may include a sacrificial material, and second layers512 may include a conductor material for forming conductor layers thatsubsequently function as the gate electrodes of the memory device.Detailed description of the material of substrate 50, and the materialand fabrication process to form stack structure 51 can be referred tothe description of substrate 20 and stack structure 21 in FIG. 2A, andis not repeated herein. In some embodiments, substrate 50 includessilicon, first layer 511 includes silicon nitride and/or silicon oxide,and second layers 512 include polysilicon.

As shown in FIG. 5A, a channel hole 52 may be formed extendingvertically through stack structure 51. The fabrication process to formchannel hole 52 may be similar to or the same as the fabrication processto form initial channel hole 22 (e.g., illustrated in FIG. 2B).Different from the formation of channel hole 222 illustrated in FIG. 2C,no offset is formed between side surfaces of first layer 511 and secondlayer 512 in channel hole 52. That is, the side surfaces of first layer511 and second layer 512 may be coplanar along the vertical direction. Ablocking material layer 531 m, a memory material layer 532 m, and atunneling material layer 533 m may be sequentially deposited over thesidewall of channel hole 52. The materials and deposition processes toform these material layers can be referred to the description ofmaterials and deposition processes of the blocking material layer, thememory material layer, and the tunneling material illustrated in FIG.2D, and are not repeated herein.

As shown in FIG. 5B, portions of blocking material layer 531 m, memorymaterial layer 532 m, and tunneling material layer 533 m may be removedto expose substrate 50. An etching process, similar to the etchingprocess illustrated in FIG. 2D, may be performed, and blocking layer531, memory layer 532, and tunneling layer 533, may be formed.

As shown in FIG. 5C, a semiconductor layer 534 and a dielectric core 59may sequentially be deposited to fill up channel hole 52 and formsemiconductor channel 54. The materials and deposition processes to formsemiconductor layer 534 and dielectric core may be referred to thedescription of materials and deposition processes to form semiconductorlayer 234 and dielectric core 29 illustrated in FIGS. 2E and 2F, and arenot repeated herein.

Referring back to FIG. 10, after the formation of the semiconductorchannel, a gate-to-gate dielectric layer is formed between adjacentconductor layers, and a second initial slit opening is formed (Operation1004). FIGS. 5D and 5E illustrate corresponding structures having agate-to-gate dielectric layer with an airgap. FIGS. 5D and 5F illustratecorresponding structures having a gate-to-gate dielectric layer withoutan airgap.

As shown in FIG. 5D, a first initial slit opening 55 can be formedextending vertically through the stack structure, and first layers 511may be removed through first initial slit openings 55 to form aplurality of lateral recesses. The formation of first initial slitopenings 55 can be referred to the formation of first initial slitopening 25 illustrated n FIG. 2G, and the formation of lateral recessesand can be referred to the formation of lateral recesses illustrated inFIG. 3A. In some embodiments, portions of block layer 531 are exposed inthe lateral recesses. Details are not repeated herein.

FIG. 5E illustrates a structure formed from the structure illustrated inFIG. 5D. In some embodiments, as shown in FIG. 5E, a gate-to-gatedielectric layer 57 and a second initial slit opening 55A can be formed.Gate-to-gate dielectric layer 57 may be located between adjacentconductor layers 58. Gate-to-gate dielectric layer 57 may include a pairof composite layers 57-1 and 57-2, and an airgap 573 between compositelayers 57-1 and 57-2. The materials, structures, and fabrication processto form gate-to-gate dielectric layer 57 and second initial slit opening55A may be referred to the description of materials, structures, andfabrication process to form gate-to-gate dielectric layer 37 and secondinitial slit opening 35A illustrated in FIG. 3A and are not repeatedherein.

FIG. 5F illustrates another structure formed from the structureillustrated in FIG. 5D. In some embodiments, as shown in FIG. 5E, agate-to-gate dielectric layer 57 and a second initial slit opening 55Bcan be formed. Gate-to-gate dielectric layer 57 may be located betweenadjacent conductor layers 58 and have no airgap between adjacentconductor layers 58. Gate-to-gate dielectric layer 57 may include acomposite layer between adjacent conductor layers 58. The materials,structures, and fabrication process to form gate-to-gate dielectriclayer 57 and second initial slit opening 55B may be referred to thedescription of materials, structures, and fabrication process to formgate-to-gate dielectric layer 37 and second initial slit opening 35Billustrated in FIG. 3B and are not repeated herein.

Referring back to FIG. 10, after the formation of gate-to-gatedielectric layers and second initial slit openings, a doped region isformed at the bottom of the second slit structure and a slit structureis formed from the second initial slit structure (Operation 1006). FIGS.5G and 5H each illustrates a respective structure.

As shown in FIGS. 5G and 5H, a doped region 56 is formed in respectivesubstrate 50, and a slit structure (e.g., 550A in FIG. 5G and 550B inFIG. 5H) is formed extending through the stack structure and exposingsubstrate 50 (e.g., the respective doped region 56). The specificfabrication processes to form doped region 56 and slit opening 550A/550Bshould be referred to the description of fabrication processes to formdoped region 36 and slit opening 350A/350B, and are not repeated herein.

Referring back to FIG. 10, after the formation of the doped region andslit structure, an insulating structure is formed in the slit structureand a source contact is formed in the insulating structure (Operation1008). FIGS. 5I and 5J each illustrates a respective structure.

As shown in FIGS. 5I and 5J, an insulating structure (e.g., 520A in FIG.5I and 520B in FIG. 5J) and a source contact 521 are formed inrespective insulating structure 520A/520B. In some embodiments, sourcecontact 521 contacts respective doped region 36. Description ofmaterials and fabrication processes to form insulating structures520A/520B and source contact 521 should be referred to the descriptionof materials and fabrication processes to form insulating structures320A/320B and source contact 521 illustrated in FIGS. 3I and 3J, and arenot repeated herein.

FIGS. 6A-6I illustrate a “gate last” method to form memory devices witha gate-to-gate dielectric layer between adjacent conductor layers fromstructure 200, according to some embodiments. Specifically, FIGS. 6A,6B, 6D, 6F, and 6H illustrate the fabrication process to form agate-to-gate dielectric layer from an entirety of each of the pluralityof first layers, and FIGS. 6A, 6C, 6E, 6G, and 6I illustrate thefabrication process to form a gate-to-gate dielectric layer from aportion of each of the plurality of first layers. In some embodiments,FIGS. 6A, 6B, 6D, 6F, and 6H illustrate the fabrication process to formmemory device 104, and FIGS. 6A, 6C, 6E, 6G, and 6I illustrate thefabrication process to form memory device 106. In this “gate last”method, first layers 211 include a dielectric material for forming thegate-to-gate dielectric layers and second layers 212 include asacrificial material for forming the conductor layers that function asgate electrodes. The dielectric material may include silicon oxideand/or silicon nitride. In some embodiments, first layers 211 includesilicon nitride. In some embodiments, second layers 212 include adifferent material than the material of first layers 211. In someembodiments, second layers 212 include polysilicon, carbon, and/ororganic films. FIG. 9D illustrates a flowchart 960 for fabricationprocesses depicted in FIGS. 6A-6I.

As shown in FIG. 6A, at the beginning of the fabrication process, theplurality of second layers are removed (Operation 962). FIG. 6Aillustrates a corresponding structure.

In some embodiments, an isotropic etching process (e.g., wet etch) isperformed to remove second layers 212 and expose blocking layer 231 andsubstrate 20. A plurality of lateral recesses 62 can be formed from theremoval of second layers 212 through first initial slit opening 25.Portions of blocking layer 231 can be exposed by lateral recesses 62.

Referring back to FIG. 9D, after the removal of second layers andformation of lateral recesses, a gate-to-gate dielectric layer is formedbetween adjacent lateral recesses and a second initial slit opening isformed (Operation 964). FIGS. 6B and 6C each illustrates a correspondingstructure.

In some embodiments, gate-to-gate dielectric layers 67 of FIGS. 6A and6B are formed by oxidizing first layers 211 through first initial slitopening 25 and lateral recesses 62. In some embodiments, to form aplurality of gate-to-gate dielectric layers 67, oxygen diffusionconcentration is controlled, such that each gate-to-gate dielectriclayer 37 includes a desired number of sub-layers of silicon oxynitrideand/or silicon oxide. The specific structure of each composite layershould not be limited by the embodiments of the present disclosure. Asecond initial slit opening (e.g., 65A in FIG. 6B and 65B in FIG. 6C)may be formed from the respective first initial slit opening (e.g., 25in FIG. 6A) by the oxidation process on first layers 211. In someembodiments, an oxidized layer 61 may be formed over substrate 20 at thebottom of second initial slit structure 65A/65B from the oxidationreaction between oxygen and substrate 20.

FIG. 6B illustrates the structure in which each gate-to-gate dielectriclayer is formed by fully oxidizing each first layer 211. As shown inFIG. 6B, an oxidation reaction may be performed to form a gate-to-gatedielectric layer 67 from the oxidation of the entire portion of eachfirst layer 211. Each gate-to-gate dielectric layer 67 may include acomposite layer that includes at least a sub-layer of siliconoxynitride, formed from the entire portion of a respective first layer211, between adjacent conductor layers that are subsequently formed. Insome embodiments, each composite layer includes at least a sub-layer ofsilicon oxynitride and at least a sub-layer of silicon oxide. In someembodiments, each composite layer includes a plurality of alternatingarranged sub-layers of silicon oxynitride and silicon oxide, such as thestructure illustrated in FIG. 8B.

FIG. 6C illustrates the structure in which a gate-to-gate dielectriclayer 67 is formed by partially oxidizing each first layer 211.Gate-to-gate dielectric layer 67 may include a pair of composite layers(e.g., 67-1 and 67-2) that are formed from the oxidation of the outsideportion, instead of the entire portion, of each first layer 211. Asshown in FIG. 6C, an oxidation reaction may be performed to form agate-to-gate dielectric layer 67 from the outside portion of each firstlayer 211. Each gate-to-gate dielectric layer 67 may include a pair ofcomposite layers (e.g., 67-1 and 67-2) formed between adjacent conductorlayers that are formed subsequently. Each composite layer may be formedfrom an outside portion of first layer 211. In some embodiments,composite layer 67-1 is formed from a top portion of first layer 211(e.g., a portion extending from the upper surface of first layer 211into the inside of first layer 211) and composite layer 67-2 is formedfrom a bottom portion of the same first layer 211 (e.g., a portionextending from the lower surface of first layer 211 into the inside offirst layer 211). The unreacted portion of first layer 211 may besandwiched or surrounded by composite layers 67-1 and 67-2, and may bereferred to as an unreacted dielectric layer 670 (e.g., consisting ofsilicon nitride). In some embodiments, gate-to-gate dielectric layer 67includes a pair of composite layers 67-1 and 67-2 and unreacteddielectric layer 670 between composite layers 67-1 and 67-2. Thethicknesses of composite layers 67-1 and 67-2, and unreacted dielectriclayer 670 may each be determined by the oxidation process, where thethickness of unreacted dielectric layer 670 is greater than zero. Insome embodiments, each composite layer 67-1/67-2 includes at least asub-layer of silicon oxynitride. In some embodiments, each compositelayer 67-1/67-2 includes at least a sub-layer of silicon oxynitride andat least a sub-layer of silicon oxide. In some embodiments, eachcomposite layer includes a plurality of alternating arranged sub-layersof silicon oxynitride and silicon oxide, such as the structureillustrated in FIG. 8B. In some embodiments, gate-to-gate dielectriclayer 67 includes a pair of composite layers 67-1 and 67-2, and theunreacted dielectric layer 670 between composite layers 67-1 and 67-2.That is, gate-to-gate dielectric layer 67 includes a sub-layer ofsilicon nitride sandwiched by two alternatingly arranged stacks ofsub-layers of silicon oxynitride and silicon oxide.

Referring back to FIG. 9D, after the formation of gate-to-gatedielectric layers, a plurality of conductor layers and a slit openingare formed (Operation 966). FIGS. 6D and 6E each illustrates acorresponding structure.

As shown in FIGS. 6D and 6E, a plurality of conductor layers 68 and arespective slit opening (e.g., 650A in FIG. 6D and 650B in FIG. 6E) isformed from the respective second initial slit opening 65A/65B. In someembodiments, a conductor material layer can be deposited into eachlateral recesses 62 to fill up the space in lateral recess 62 throughrespective second initial slit opening 65A/65B, and a recess etch (e.g.,dry and/or wet etch) can be performed to remove any excess conductormaterial layer and portions of composite layer 67-1/67-2 on the sidewallof second initial slit opening 65A/65B, forming respective conductorlayers 68 and respective slit opening 650A/650B. In some embodiments,conductor layers 68 includes tungsten, copper, aluminum, cobalt,silicides, doped and/or polysilicon. In some embodiments, an adhesivelayer 624 is deposited in lateral recesses 62 through respective secondinitial slit openings before the deposition of conductor material layer,e.g., to improve the adhesion between the conductor material layer andgate-to-gate dielectric layer 67. In some embodiments, adhesion layer624 includes titanium (Ti) and/or titanium nitride (TiN). In someembodiments, the conductor material layers and adhesive layers 624 areeach deposited by a suitable method such as one or more of CVD, ALD,LPCVD, and/or PVD.

Referring back to FIG. 9D, after the formation of conductor layers, adoped region is formed in the substrate at a bottom of the slit openingand an insulating structure is formed in the slit opening (Operation968). FIGS. 6F and 6G each illustrates a corresponding structure.

As shown in FIGS. 6F and 6G, a respective doped region 66 can be formedin substrate 20. Doped region 16 may include a suitable doped (e.g.,P-type or N-type) semiconductor region formed in substrate 10 and isopposite from the polarity of substrate 20. A suitable doping process,such as ion implantation, can be performed to form doped region 66. Insome embodiments, doped region 66 includes doped silicon.

A respective insulating structure (e.g., 620A in FIG. 6F and 620B inFIG. 6G) can be formed to insulate respective conductor layers 68 fromsubsequently-formed source contacts. In some embodiments, insulatingstructure 620A/620B each covers the sidewall of the respective slitopening and exposes substrate 20 (e.g., respective doped region 66). Insome embodiments, insulating structure 620A covers the side surfaces ofcomposite layers of gate-to-gate dielectric layer 67, conductor layers68, and adhesion layer 624. In some embodiments, insulating structure620B covers the side surfaces of composite layers of gate-to-gatedielectric layer 67, unreacted dielectric layer 670 of gate-to-gatedielectric layer 67, conductor layers 68, and adhesion layer 624. Toform insulating structure 620A/620B, a suitable insulating material canbe deposited to cover the sidewall of the respective slit opening650A/650B, and a suitable recess etch (e.g., dry etch and/or wet etch)can be performed to remove excess portions of the insulating material onthe sidewall and bottom of slit opening 650A/650B. Respective oxidizedlayer 61 can also be removed by the recess etching process. Insulatingstructure 620A/620B can be formed in slit opening 650A/650B. In someembodiments, insulating structure 120 includes silicon oxide and isdeposited by any one of CVD, ALD, LPCVD, and/or PVD. In variousembodiments, the order to form respective insulating structure 620A/620Band doped region 66 can vary based on different fabrication operationsand should not be limited by the embodiments of the present disclosure.

Referring back to FIG. 9D, after the formation of insulating structuresand doped regions, a source contact is formed in the insulatingstructure (Operation 970). FIGS. 6H and 6I each illustrates acorresponding structure.

As shown in FIGS. 6H and 6I, a source contact 621 is formed inrespective insulating structure 620A/620B. Source contact 621 maycontact respective doped region 66 and form an electrical connectionwith semiconductor channels 24 through doped region 66 and substrate 20.Source contact 621 can include one or more of tungsten, cobalt, copper,aluminum, silicides, and/or doped polysilicon, and can be deposited byone or more of CVD, PVD, and/or ALD. A suitable CMP and/or recess etchcan be performed to remove the excess materials of insulating structure620A/620B and source contact 621.

In some embodiments, the “gate last” method is also employed to form amemory device that has a semiconductor channel with no lateral portions,e.g., extending along the vertical direction consistently. For example,to form the memory device, a semiconductor channel similar to or thesame as semiconductor channel 54 (e.g., illustrated in FIG. 5C) can beformed in a stack structure. The stack structure, different from stackstructure 51, can be have a plurality of alternatingly arranged firstlayers of a dielectric material layer and second layers of a sacrificialmaterial layer, similar to or the same as the stack structureillustrated in FIGS. 6A-6I. In some embodiments, the first layersinclude silicon nitride and the second layers include a differentmaterial than the first layers, such as polysilicon, carbon, and/ororganic films. The second layers can be removed to form a plurality oflateral recesses, similar to the fabrication operation illustrated inFIG. 6A. The first layer may then be oxidized using an oxidationreaction similar to the oxidation process illustrated in FIGS. 6B and 6Cto form a plurality of gate-to-gate dielectric layers. The stackstructure may further be processed, using the fabrication processesillustrated in FIGS. 6D-6I, to form other parts, e.g., source contacts,insulating structures, and conductor layers. A detailed description ofthe material and fabrication process to form the memory device can bereferred to the description of FIGS. 5A-5J and FIGS. 6A-6I, and is thusnot repeated herein.

In various embodiments, based on the material of the first layers and/orsecond layers, the gate-to-gate dielectric layer may include differentmaterials than the materials introduced in the present disclosure. Byusing the methods of the present disclosure, the first layers and/or thesecond layers can undergo a suitable reaction (e.g., oxidizing and/ornitriding reaction) to form at least a sub-layer of a high-k dielectricmaterial in the respective gate-to-gate dielectric layer. For example,x81 may include hafnium oxide (HfO_(x)) and x82 may include hafniumoxynitride (HfO_(x)N_(y), e.g., HfON). In some embodiments, gate-to-gatedielectric layer 17 of memory devices 102 and 104 may be formed bydepositing hafnium oxide to fill up the lateral recesses which areformed by the removal of first layers 211, and performing an oxidizingand/or nitriding process on the hafnium oxide between conductor layers18 to form at least a sub-layer of hafnium oxynitride in gate-to-gatedielectric layer 17. In some embodiments, in a “gate first” method,second layers 212 includes hafnium and gate-to-gate dielectric layer 17of memory devices 101, 103, 105, and 106 (e.g., each formed by a “gatefirst” method) includes at least a sub-layer of hafnium oxynitride. Insome embodiments, in a “gate last” method, first layers 211 includeshafnium and gate-to-gate dielectric layer 17 of memory devices 104 and106 (e.g., each formed by a “gate last” method) includes at least asub-layer of hafnium oxynitride. The specific materials of thegate-to-gate dielectric layer should not be limited by the embodimentsof the present disclosure.

In some embodiments, a method for forming a 3D memory device includesthe following operations. First, an initial channel hole is formed in astack structure of a plurality first layers and a plurality of secondlayers alternatingly arranged over a substrate. An offset is formedbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole. Asemiconductor channel is formed by filling the channel hole with achannel-forming structure. The semiconductor channel may have a memorylayer including a plurality of first memory portions each surrounding abottom of a respective second layer and a plurality of second memoryportions each connecting adjacent first memory portions. Further, theplurality of second memory portions is removed to retain the pluralityof first memory portions. The plurality of first memory portions may bedisconnected from one another. In some embodiments, a plurality ofconductor layers are formed from the plurality of second layers and agate-to-gate dielectric layer is formed between the adjacent conductorlayers. The gate-to-gate dielectric layer includes at least onesub-layer of silicon oxynitride.

In some embodiments, removing the plurality of second memory portionsincludes forming a first initial slit opening extending through thestack structure and exposing the substrate, removing the plurality offirst layers through the first initial slit to form a plurality oflateral recesses that expose portions of the semiconductor channel, andperforming an etching process on the exposed portions of thesemiconductor channel through the plurality of lateral recesses and thefirst initial slit opening to remove the plurality of second memoryportions.

In some embodiments, filling the channel hole with a channel-formingstructure includes forming a blocking layer over a sidewall of thechannel hole, forming the memory layer over the blocking layer, forminga tunneling layer over the memory layer, forming a semiconductor layerover the tunneling layer, and forming a dielectric core over thesemiconductor layer to fill up the channel hole. In some embodiments,removing the plurality of second memory portions includes removing aportion of the blocking layer over each one of the plurality of thesecond memory portions and removing the plurality of second memoryportions to expose a portion of the tunneling layer under each one ofthe plurality of second memory portions.

In some embodiments, forming the plurality of conductor layers, thegate-to-gate dielectric layer, and a second initial slit opening includeforming a composite layer from a portion of each of the plurality ofsecond layers, a remaining portion of the respective second layerforming a respective conductor layer, a pair of composite layers on theadjacent conductor layers and facing each other forming the gate-to-gatedielectric layer, the first initial slit opening forming a secondinitial slit opening, the composite layer having at least one sub-layerof silicon oxynitride.

In some embodiments, the plurality of second layers include polysiliconand forming the composite layer includes performing, through the firstinitial slit opening and the plurality of lateral recesses, one or moreof an oxidation reaction and a nitriding reaction on the plurality ofsecond layers. A reacted portion of each of the plurality of secondlayers may form the respective composite layer and an unreacted portionof each of the plurality of second layers may form the respectiveconductor layer.

In some embodiments, a composite layer is formed from each of a topportion and a bottom portion of the respective second layer.

In some embodiments, forming the gate-to-gate dielectric layer furtherincludes forming an airgap between the pair of composite layers.

In some embodiments, forming the composite layer includes controllingthe oxygen diffusion concentration such that the composite layerincludes the at least one sub-layer of silicon oxynitride.

In some embodiments, forming the composite layer further includescontrolling the oxygen diffusion concentration such that the compositelayer includes at least one sub-layer of silicon oxynitride and at leastone sub-layer of silicon oxide.

In some embodiments, forming the composite layer further includescontrolling the oxygen diffusion concentration such that the compositelayer includes a plurality of alternatingly arranged sub-layers ofsilicon oxynitride and sub-layers of silicon oxide.

In some embodiments, the plurality of second layers include a conductormaterial and forming the gate-to-gate dielectric structure includesfilling each of the plurality of lateral recesses with a gate-to-gatedielectric layer. The plurality of second layers may form the pluralityof conductor layers and the gate-to-gate dielectric layer may include atleast one sub-layer of silicon oxynitride.

In some embodiments, the gate-to-gate dielectric layer and a secondinitial slit opening are formed by depositing a layer of silicon nitrideto fill up each of the plurality of lateral recesses, oxidizing thelayer of silicon nitride by controlling oxygen diffusion concentrationsuch that the gate-to-gate dielectric layer includes the at least onesub-layer of silicon oxynitride, and the first initial slit openingforming the second initial slit opening.

In some embodiments, forming the gate-to-gate dielectric layer furtherincludes controlling oxygen diffusion concentration such that thegate-to-gate dielectric layer includes at least one sub-layer of siliconoxynitride and at least one sub-layer of silicon oxide between adjacentsecond layers.

In some embodiments, forming the gate-to-gate dielectric layer furtherincludes controlling oxygen diffusion concentration such that thegate-to-gate dielectric layer includes a plurality of alternatinglyarranged sub-layers of silicon oxynitride and sub-layers of siliconoxide between adjacent second layers.

In some embodiments, forming the offset includes removing a portion ofthe side surface of each one of the plurality of first layers on thesidewall of the initial channel hole.

In some embodiments, removing the portion of the side surface of eachone of the plurality of first layers includes performing a recessetching process that selectively etches the plurality of first layersover the plurality of second layers.

In some embodiments, the plurality of first layers and the plurality ofsecond layers are formed by alternatingly depositing a plurality offirst material layers and a plurality of second material layers over thesubstrate to form an initial stack structure over the substrate. Theplurality of first material layers may have a different etchingselectivity than the plurality of second material layers. In someembodiments, the plurality of first layers and the plurality of secondlayers are formed by repetitively etching the plurality of firstmaterial layers and the plurality of second material layers to form thestack structure having the plurality of first layers and the pluralityof second layers arranged in a staircase structure.

In some embodiments, depositing the plurality of first material layersincludes depositing at least one of a silicon nitride material layer, asilicon oxide material layer, or a silicon oxynitride material layer.

In some embodiments, the method further includes forming a doped regionin the substrate at a bottom of the second initial slit opening, forminga slit opening from the second initial slit opening by removing portionsof the composite layer to expose the plurality of conductor layers on asidewall of the slit opening and to expose the substrate at a bottom ofthe slit opening, and forming an insulating structure in the slitopening. The insulating structure may be over the exposed portions ofthe plurality of conductor layers and exposing the substrate at thebottom of the slit opening. The method also includes forming a sourcecontact in the insulating structure and in contact with the dopedregion.

In some embodiments, forming an insulating structure in the slit openingincludes depositing a layer of silicon oxide layer covering the exposedportions of the plurality of conductor layers and the gate-to-gatedielectric layer between adjacent conductor layers. In some embodiments,forming the source contact includes depositing at least one of tungsten,cobalt, copper, aluminum, polysilicon, doped silicon, or silicides inthe insulating structure.

In some embodiments, a method for forming a 3D memory device includesthe following operations. First, an initial channel hole is formed in astack structure of a plurality first layers and a plurality of secondlayers alternatingly arranged over a substrate. An offset is formedbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole. Asemiconductor channel is further formed by filling the channel hole witha channel-forming structure. The semiconductor channel may have a memorylayer including a plurality of first memory portions each surrounding abottom of a respective second layer and a plurality of second memoryportions each connecting adjacent first memory portions. Further, theplurality of second memory portions are removed to retain the pluralityof first memory portions. The plurality of first memory portions may bedisconnected from one another. A plurality of conductor layers areformed from the plurality of second layers and a gate-to-gate dielectricstructure that insulates each one of the plurality of conductor layersfrom one another is formed. The gate-to-gate dielectric structure mayinclude a plurality of composite layers separated by the plurality ofconductor layers. Each one of the plurality of composite layers mayinclude a stack of sub-layers having at least one sub-layer of siliconoxynitride.

In some embodiments, removing the plurality of second memory portionsincludes forming a first initial slit opening extending through thestack structure and exposing the substrate, removing the plurality offirst layers through the first initial slit to form a plurality oflateral recesses that expose portions of the semiconductor channel, andperforming an etching process on the exposed portions of thesemiconductor channel through the plurality of lateral recesses and thefirst initial slit opening to remove the plurality of second memoryportions.

In some embodiments, filling the channel hole with a channel-formingstructure includes forming a blocking layer over a sidewall of thechannel hole, forming the memory layer over the blocking layer, forminga tunneling layer over the memory layer, forming a semiconductor layerover the tunneling layer, and forming a dielectric core over thesemiconductor layer to fill up the channel hole. In some embodiments,removing the plurality of second memory portions includes removing aportion of the blocking layer over each one of the plurality of thesecond memory portions and removing the plurality of second memoryportions to expose a portion of the tunneling layer under each one ofthe plurality of second memory portions.

In some embodiments, forming the plurality of conductor layers, theplurality of composite layers, and a second initial slit opening includeforming the plurality of composite layers from a top portion and abottom portion of each of the plurality of second layers, the middleportion between the top portion and the bottom portion forming arespective conductor layer, the first initial slit opening forming asecond initial slit opening.

In some embodiments, the plurality of second layers include polysiliconand forming the plurality of composite layers includes performing,through the first initial slit opening and the plurality of lateralrecesses, one or more of an oxidation reaction and a nitriding reactionon the plurality of second layers. Reacted top and bottom portions ofeach of the plurality of second layers may form the respective compositelayers and an unreacted portion between the reacted top and bottomportions of each of the plurality of second layers may form therespective conductor layer.

In some embodiments, the method further includes forming an airgapbetween composite layers on adjacent conductor layers and facing eachother.

In some embodiments, forming the composite layer further includescontrolling the oxygen diffusion concentration such that the compositelayer includes at least one sub-layer of silicon oxynitride and at leastone sub-layer of silicon oxide.

In some embodiments, forming the composite layer further includescontrolling the oxygen diffusion concentration such that the compositelayer includes a plurality of alternatingly arranged sub-layers ofsilicon oxynitride and sub-layers of silicon oxide.

In some embodiments, the plurality of second layers include a conductormaterial and forming the gate-to-gate dielectric structure includesfilling each of the plurality of lateral recesses with one of theplurality of composite layers. The plurality of second layers form theplurality of conductor layers.

In some embodiments, the plurality of composite layers and a secondinitial slit opening are formed by depositing a layer of silicon nitrideto fill up each of the plurality of lateral recesses, oxidizing thelayer of silicon nitride by controlling oxygen diffusion concentrationsuch that each one of the plurality of composite layers includes the atleast one sub-layer of silicon oxynitride, and the first initial slitopening forming the second initial slit opening.

In some embodiments, forming the plurality of composite layers furtherincludes controlling oxygen diffusion concentration such that the eachone of the composite layers includes at least one sub-layer of siliconoxynitride and at least one sub-layer of silicon oxide between adjacentsecond layers.

In some embodiments, forming the plurality of composite layers furtherincludes controlling oxygen diffusion concentration such that each oneof the plurality of composite layers includes a plurality ofalternatingly arranged sub-layers of silicon oxynitride and sub-layersof silicon oxide between adjacent second layers.

In some embodiments, forming the offset includes removing a portion ofthe side surface of each one of the plurality of first layers on thesidewall of the initial channel hole.

In some embodiments, removing the portion of the side surface of eachone of the plurality of first layers includes performing a recessetching process that selectively etches the plurality of first layersover the plurality of second layers.

In some embodiments, the plurality of first layers and the plurality ofsecond layers are formed by alternatingly depositing a plurality offirst material layers and a plurality of second material layers over thesubstrate to form an initial stack structure over the substrate. Theplurality of first material layers may have a different etchingselectivity than the plurality of second material layers. In someembodiments, the plurality of first layers and the plurality of secondlayers are formed by repetitively etching the plurality of firstmaterial layers and the plurality of second material layers to form thestack structure having the plurality of first layers and the pluralityof second layers arranged in a staircase structure.

In some embodiments, depositing the plurality of first material layersincludes depositing at least one of a silicon nitride material layer, asilicon oxide material layer, or a silicon oxynitride material layer.

In some embodiments, the method further including forming a doped regionin the substrate at a bottom of the second initial slit opening, forminga slit opening from the second initial slit opening by removing portionsof the composite layer to expose the plurality of conductor layers on asidewall of the slit opening and to expose the substrate at a bottom ofthe slit opening, forming an insulating structure in the slit opening,the insulating structure being over the exposed portions of theplurality of conductor layers and exposing the substrate at the bottomof the slit opening, and forming a source contact in the insulatingstructure and in contact with the doped region.

In some embodiments, forming an insulating structure in the slit openingincludes depositing a layer of silicon oxide layer covering the exposedportions of the plurality of conductor layers and the gate-to-gatedielectric layer between adjacent conductor layers. In some embodiments,forming the source contact includes depositing at least one of tungsten,cobalt, copper, aluminum, polysilicon, doped silicon, or silicides inthe insulating structure.

In some embodiments, a 3D memory device includes a stack structurehaving a plurality of conductor layers insulated from one another by agate-to-gate dielectric structure. The gate-to-gate dielectric structuremay include at least a sub-layer of silicon oxynitride between adjacentconductor layers along a vertical direction perpendicular to a topsurface of the substrate. The 3D memory device may also include asemiconductor channel extending from a top surface of the stackstructure to the substrate. The semiconductor channel may include amemory layer that has a plurality of memory portions each surrounding abottom of a respective conductor layer and each being disconnected fromone another. The 3D memory device may further include a source structureextending from the top surface of the stack structure to the substrate.

In some embodiments, the plurality of memory portions each includes avertical portion along the vertical direction and at least one lateralportion along a lateral direction parallel to the top surface of thesubstrate. The vertical portion and the at least one lateral portion maypartially surround the respective conductor layer vertically andlaterally.

In some embodiments, the gate-to-gate dielectric structure includes agate-to-gate dielectric layer between adjacent conductor layers. Thegate-to-gate dielectric layer may include a pair of composite layers onthe adjacent conductor layers and an airgap between the pair ofcomposite layers. The pair of composite layers may each have at least asub-layer of silicon oxynitride.

In some embodiments, the pair of composite layers each includes at leasta sub-layer of silicon oxide and a sub-layer of silicon oxynitride.

In some embodiments, the pair of composite layers each includes aplurality of alternatingly arranged sub-layers of silicon oxide andsub-layers of silicon oxynitride.

In some embodiments, each composite layer is located between ends of therespective vertical portion of each of the plurality of memory portionsalong the vertical direction.

In some embodiments, the gate-to-gate dielectric structure includes aplurality of composite layers each between adjacent conductor layer andincluding at least a sub-layer of silicon oxynitride.

In some embodiments, each of the plurality of composite layers includesat least a sub-layer of silicon oxide and a sub-layer of siliconoxynitride

In some embodiments, each of the plurality of composite layers includesa plurality of alternatingly arranged sub-layers of silicon oxide andsub-layers of silicon oxynitride.

In some embodiments, along a radial direction from a sidewall of thesemiconductor channel to a center of the semiconductor channel, thesemiconductor channel includes a blocking layer, the plurality of memoryportions over the blocking layer, a tunneling layer over the pluralityof memory portions, a semiconductor layer over the tunneling layer, anda dielectric core over the semiconductor layer.

In some embodiments, the blocking layer includes at least one of a firstblocking layer and a second blocking layer, the first blocking layerincluding one or more of aluminum oxide (AlO), hafnium oxide (HfO₂),lanthanum oxide (LaO₂), yttrium oxide (Y₂O₃), tantalum oxide (Ta₂O₅),silicates thereof, nitrogen-doped compounds thereof, or alloys thereof,the second blocking layer including one or more of silicon oxide,silicon oxynitride, and silicon nitride. In some embodiments, the memorylayer includes a charge-trapping material that includes at least one oftungsten, molybdenum, tantalum, titanium, platinum, ruthenium, alloysthereof, nanoparticles thereof, silicides thereof, polysilicon,amorphous silicon, SiN, or SiON. In some embodiments, the tunnelinglayer includes at least one of SiO, SiN, SiON, dielectric metal oxides,dielectric metal oxynitride, dielectric metal silicates, or alloysthereof. In some embodiments, the semiconductor layer includes at leastone of a one-element semiconductor material, a III-V compoundsemiconductor material, a II-VI compound semiconductor material, or anorganic semiconductor material. In some embodiments, the dielectric coreincludes SiO.

In some embodiments, the plurality of conductor layers each including alayer of one or more of W, Co, Al, doped silicon, silicides, and acombination thereof. In some embodiments, the source structure eachincludes an insulating structure and a source contact in the insulatingstructure and conductively in contact with the substrate. The insulatingstructure may include silicon oxide, and the source contact includingone or more of W, Co, Al, doped silicon, silicides, and a combinationthereof.

The foregoing description of the specific embodiments will so reveal thegeneral nature of the present disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

Embodiments of the present disclosure have been described above with theaid of functional building blocks illustrating the implementation ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed.

The Summary and Abstract sections may set forth one or more but not allexemplary embodiments of the present disclosure as contemplated by theinventor(s), and thus, are not intended to limit the present disclosureand the appended claims in any way.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A method for forming a three-dimensional (3D)memory device, comprising: forming an initial channel hole in a stackstructure of a plurality of first layers and a plurality of secondlayers alternatingly arranged over a substrate; forming an offsetbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole; forming asemiconductor channel by filling the channel hole with a channel-formingstructure, the semiconductor channel having a memory layer comprising aplurality of first memory portions each surrounding a bottom of arespective second layer and a plurality of second memory portions eachconnecting adjacent first memory portions; removing the plurality offirst layers to form a plurality of lateral recesses that exposeportions of the semiconductor channel; removing the plurality of secondmemory portions through the plurality of lateral recesses to retain theplurality of first memory portions, the plurality of first memoryportions being disconnected from one another; forming a plurality ofconductor layers from the plurality of second layers; and forming agate-to-gate dielectric layer between the adjacent conductor layers, thegate-to-gate dielectric layer comprising at least one sub-layer ofsilicon oxynitride.
 2. The method of claim 1, wherein removing theplurality of second memory portions comprises: forming a first initialslit opening extending through the stack structure and exposing thesubstrate; and performing an etching process on the exposed portions ofthe semiconductor channel through the plurality of lateral recesses andthe first initial slit opening to remove the plurality of second memoryportions.
 3. The method of claim 2, wherein forming the plurality ofconductor layers, the gate-to-gate dielectric layer, and a secondinitial slit opening comprise: forming a composite layer from a portionof each of the plurality of second layers, a remaining portion of therespective second layer forming a respective conductor layer, a pair ofcomposite layers on the adjacent conductor layers and facing each otherforming the gate-to-gate dielectric layer, the first initial slitopening forming a second initial slit opening, the composite layerhaving at least one sub-layer of silicon oxynitride.
 4. The method ofclaim 3, wherein the plurality of second layers comprise polysilicon andforming the composite layer comprises: performing, through the firstinitial slit opening and the plurality of lateral recesses, one or moreof an oxidation reaction and a nitriding reaction on the plurality ofsecond layers, a reacted portion of each of the plurality of secondlayers forming the respective composite layer, an unreacted portion ofeach of the plurality of second layers forming the respective conductorlayer.
 5. The method of claim 4, wherein forming the gate-to-gatedielectric layer further comprises forming an airgap between the pair ofcomposite layers.
 6. The method of claim 5, wherein forming thecomposite layer comprises controlling an oxygen diffusion concentrationsuch that the composite layer comprises the at least one sub-layer ofsilicon oxynitride.
 7. The method of claim 6, wherein forming thecomposite layer further comprises controlling the oxygen diffusionconcentration such that the composite layer comprises at least onesub-layer of silicon oxynitride and at least one sub-layer of siliconoxide.
 8. The method of claim 7, wherein forming the composite layerfurther comprises controlling the oxygen diffusion concentration suchthat the composite layer comprises a plurality of alternatingly arrangedsub-layers of silicon oxynitride and sub-layers of silicon oxide.
 9. Themethod of claim 3, wherein the plurality of second layers comprise aconductor material and forming the gate-to-gate dielectric structurecomprises filling each of the plurality of lateral recesses with agate-to-gate dielectric layer, the plurality of second layers formingthe plurality of conductor layers, the gate-to-gate dielectric layercomprising at least one sub-layer of silicon oxynitride.
 10. The methodof claim 9, wherein the gate-to-gate dielectric layer and a secondinitial slit opening are formed by: depositing a layer of siliconnitride to fill up each of the plurality of lateral recesses; oxidizingthe layer of silicon nitride by controlling oxygen diffusionconcentration such that the gate-to-gate dielectric layer comprises theat least one sub-layer of silicon oxynitride; and the first initial slitopening forming the second initial slit opening.
 11. The method of claim10, wherein forming the gate-to-gate dielectric layer further comprisescontrolling oxygen diffusion concentration such that the gate-to-gatedielectric layer comprises at least one sub-layer of silicon oxynitrideand at least one sub-layer of silicon oxide between adjacent secondlayers.
 12. The method of claim 11, wherein forming the gate-to-gatedielectric layer further comprises controlling an oxygen diffusionconcentration such that the gate-to-gate dielectric layer comprises aplurality of alternatingly arranged sub-layers of silicon oxynitride andsub-layers of silicon oxide between adjacent second layers.
 13. Themethod of claim 1, wherein forming the offset comprises removing aportion of the side surface of each one of the plurality of first layerson the sidewall of the initial channel hole by a recess etching processthat selectively etches the plurality of first layers over the pluralityof second layers.
 14. A method for forming a three-dimensional (3D)memory device, comprising: forming an initial channel hole in a stackstructure of a plurality of first layers and a plurality of secondlayers alternatingly arranged over a substrate; forming an offsetbetween a side surface of each one of the plurality of first layers anda side surface of each one of the plurality of second layers on asidewall of the initial channel hole to form a channel hole; forming asemiconductor channel by filling the channel hole with a channel-formingstructure, the semiconductor channel having a memory layer comprising aplurality of first memory portions each surrounding a bottom of arespective second layer and a plurality of second memory portions eachconnecting adjacent first memory portions; removing the plurality offirst layers to form a plurality of lateral recesses that exposeportions of the semiconductor channel; removing the plurality of secondmemory portions through the lateral recesses to retain the plurality offirst memory portions, the plurality of first memory portions beingdisconnected from one another; forming a plurality of conductor layersfrom the plurality of second layers; and forming a gate-to-gatedielectric structure that insulates each one of the plurality ofconductor layers from one another, the gate-to-gate dielectric structurecomprising a plurality of composite layers separated by the plurality ofconductor layers, each one of the plurality of composite layerscomprising a stack of sub-layers having at least one sub-layer ofsilicon oxynitride.
 15. The method of claim 14, wherein removing theplurality of second memory portions comprises: forming a first initialslit opening extending through the stack structure and exposing thesubstrate; and performing an etching process on the exposed portions ofthe semiconductor channel through the plurality of lateral recesses andthe first initial slit opening to remove the plurality of second memoryportions.
 16. The method of claim 15, wherein the plurality of secondlayers comprise polysilicon and forming the plurality of compositelayers comprises: performing, through the first initial slit opening andthe plurality of lateral recesses, one or more of an oxidation reactionand a nitriding reaction on the plurality of second layers, reacted topand bottom portions of each of the plurality of second layers formingthe respective composite layers, an unreacted portion between thereacted top and bottom portions of each of the plurality of secondlayers forming the respective conductor layer.
 17. The method of claim16, further comprising forming an airgap between composite layers onadjacent conductor layers and facing each other.
 18. A three-dimensional(3D) memory device, comprising: a stack structure comprising a pluralityof conductor layers insulated from one another by a gate-to-gatedielectric structure, wherein the gate-to-gate dielectric structurecomprises a composite layer comprising at least a sub-layer of siliconoxynitride between adjacent conductor layers along a vertical directionperpendicular to a top surface of a substrate, the composite layerscovering a surface of the respective conductor layer; a semiconductorchannel extending from a top surface of the stack structure to thesubstrate, wherein the semiconductor channel comprises a memory layercomprising a plurality of memory portions each surrounding a bottom of arespective conductor layer and each being disconnected from one another;and a source structure extending from the top surface of the stackstructure to the substrate.
 19. The 3D memory device of claim 18,wherein the plurality of memory portions each comprises a verticalportion along the vertical direction and at least one lateral portionalong a lateral direction parallel to the top surface of the substrate,the vertical portion and the at least one lateral portion partiallysurrounding the respective conductor layer vertically and laterally; andthe gate-to-gate dielectric structure comprises a gate-to-gatedielectric layer between adjacent conductor layers, the gate-to-gatedielectric layer comprising a pair of composite layers on the adjacentconductor layers and an airgap between the pair of composite layers, thepair of composite layers each having at least a sub-layer of siliconoxynitride between a pair of sub-layers of silicon oxide.
 20. The 3Dmemory device of claim 19, wherein the gate-to-gate dielectric structurecomprises a plurality of composite layers each between adjacentconductor layers and comprising at least a sub-layer of siliconoxynitride.